1. Field of the Invention
This disclosure relates generally to estimating downhole formation pressures.
2. Description of the Related Art
Formation testers are used to measure formation pressures at discrete depths to determine pressure gradients for zones of interest. The pressure gradients are used to identify fluid types and to determine hydraulic connectivity between wells. Pressure gradient quality depends upon the accuracy of the formation pressure measurement. Pressure measurement values are also used to estimate the level of pressure depletion, to check connectivity between different zones, and to control the equivalent circulation density (ECD) during drilling of the wells. Therefore, making accurate pressure measurement at each depth is highly desirable.
Wells are commonly drilled wherein the pressure in the well due to the weight of the drilling mud column is greater than the connate formation pressure. Such a drilling is referred to as drilling under an overbalanced pressure or an overburdened condition. During overbalanced drilling, the drilling mud invades or penetrates the permeable rocks (formation) penetrated by the well. This mud filtrate invasion causes pressure supercharging, which is defined as the increased pressure observed at the wellbore sandface (i.e., at the wellbore wall). Pressure supercharging typically is a function of the mudcake quality (permeability and thickness), pressure overbalance, and formation permeability. The time period for which a formation is exposed to the overbalanced pressure also can also affect the amount of the supercharging. The formation pressure measurements are often affected by the amount of supercharging. Therefore, it is desirable to eliminate the pressure supercharging effect by subtracting the supercharged pressure from the measured pressure. One method for eliminating the supercharging effect is to pump the formation fluid from the formation for a relatively long time period with a large pressure drop, especially in low permeability formations. Such a method is generally not practical, especially in logging-while-drilling (LWD) environments, If the mudcake is leaky, even pumping for a long time may not necessarily eliminate the supercharging effect. Thus, estimating the amount of pressure supercharging offers a viable alternative.